1. Field of the Invention
The present invention relates to the field of power supplies and, in particular, to a dual-mode power supply.
2. Description of the Related Art
Efficiently supplying power to the various electrical loads within a computing device has proven to be challenging. In general, power from a wall outlet is converted in the “silver box” and is delivered to devices within the computer at an industry-standard 12 Volt (V) direct current (DC) power rail. The power is then converted locally at each device from 12V DC to a voltage level that is useful for the device, for example, 1.0V, 3.3V, or 5.1V.
One device within a computer that receives this standard 12V power is a graphics add-in card that typically includes at least one graphics processing unit (GPU). The 12V power is converted locally using a graphics card power supply. However, designers of graphics card power supplies are faced with unique challenges related to power conversion and power transport.
Two specific power conversion challenges are known as the load step and the load release problems. A load step is categorized by a load that changes very quickly from idle to full-power. A load release is categorized by an opposite load change. GPUs often exhibit load step and load release events when users of the computer toggle between a three-dimensional (3D) game that requires high-definition (HD) graphics processing to be performed by the GPU and a GPU-idle condition, such as browsing the web or checking e-mail. For these reasons, GPU power supplies must have a fast transient response to handle these step-like changes in load.
As is known, the time needed to complete the transient portion of a response is based on the level of inductance of the inductor within the power supply. The level of inductance limits the rate of current change, so a fast-switching power supply (e.g., a power supply with a fast transient response) usually has an inductor with a relatively small inductance (e.g., 120-150 nH). On the other hand, overshoot events, characterized by current spikes to high levels for short periods of time, militate for using inductors with larger inductances. Within GPUs, overshoot events occur because GPUs exhibit frequent load release events. For example, if the output voltage is 1.0V, then a difference of 11V is realized between the input voltage (12V) and the output voltage (1V). The 11V drop helps to get current into the inductor on a load step. However, when the load is released, there is only 1V of output voltage to deplete the energy from the inductor, causing an overshoot event. Each time that the load is released, over the course of several months or years of frequent overshoot events, there may be a small amount of GPU degradation. Over time, these small degradations may cause the device to fail. Therefore, in GPU designs, there is an optimization problem when determining the proper switching frequency, inductor and capacitor values, area requirements, and dynamic load capabilities.
Power transport challenges typically relate to transporting power from the graphics card power supply located on the add-in card to the GPU itself. The GPU is typically placed somewhere near the center of the add-in card and can be surrounded on three sides by memory interfaces and on the fourth side by peripheral interfaces, e.g., Peripheral Component Interconnect Express (PCIe) and other interfaces. The power supply is generally placed to one end of the add-in card (away from the GPU) to lessen the amount of electromagnetic noise affecting the analog circuit around the GPU interfaces. Because of the memory and other interfaces surrounding the GPU, there simply is not sufficient space around the GPU for power transport connections. In many prior art configurations, then, power is supplied through the printed circuit board (PCB) over one or more copper layers of the PCB. However, with current requirements for GPUs exceeding 250 Amperes (A), a large amount of copper is used to transport the current to the GPU through layers of the PCB. As current requirements continue to rise, it is becoming increasing more evident that supplying the power through the PCB is not a viable solution.
Some conventional power supply designs utilize multiple phases to convert the power. Each phase of the power supply may work in parallel with other phases to convert power. In most cases, those chips will go into a server/client mode and become a “normal” multiphase power supply. As is known, components of a power supply are getting too big, and using multiple phases allows designers to maintain higher switching frequencies with components that are not overly expensive. For example, in a GPU that is operating at 250 A, a typical arrangement may utilize approximately six to eight phases. In conventional systems, each phase of the power supply is built using a fast-switching topology, which is a relatively small design because of the relatively small inductor within each phase. The advantage of using multiple phases in a power supply design, where each phase is fast-switching, is that the power supply is able to handle the load step and load release events quickly. However, fast-switching phase topologies are inefficient.
To overcome the efficiency problems with a fast-switching multi-phase design, designers have implemented multi-stage phases for power conversion. The first stage converts the voltage from the original input voltage to an intermediate voltage; for example, from 12V to 5V. Then, the second stage converts power from the intermediate voltage to the final output voltage. In one prior art design, the first stage may be a high-efficiency, slow-switching stage, and the second stage may be a low-efficiency, fast-switching stage. The two-stage design has a generally fast-transient response because the second stage is implemented using a fast-switching topology. Additionally, because the second stage operates from lower input voltages (e.g., 5V instead of 12V), the two-stage design has higher efficiency than a single-stage topology and can use smaller, less lossy transistors. A larger duty cycle and high frequency of a two-stage design allows for lower overshoot problems. However, the two-stage design has inherent drawbacks. Importantly, the design requires a relatively large amount of space since the entire current is converted in both stages. Thus, all the power paths carry all of the current, regardless of how much efficiency and what dynamics are needed.
As the foregoing illustrates, there is a need in the art for an improved power supply design.